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.Dd Aug 28, 2012
.Dt ZPOOL-FEATURES 7
.Os
.Sh NAME
.Nm zpool-features
.Nd ZFS pool feature descriptions
.Sh DESCRIPTION
ZFS pool on\-disk format versions are specified via "features" which replace
the old on\-disk format numbers (the last supported on\-disk format number is
28).
To enable a feature on a pool use the
.Cm upgrade
subcommand of the
.Xr zpool 8
command, or set the
.Sy feature@feature_name
property to
.Ar enabled .
.Pp
The pool format does not affect file system version compatibility or the ability
to send file systems between pools.
.Pp
Since most features can be enabled independently of each other the on\-disk
format of the pool is specified by the set of all features marked as
.Sy active
on the pool. If the pool was created by another software version this set may
include unsupported features.
.Ss Identifying features
Every feature has a guid of the form
.Sy com.example:feature_name .
The reverse DNS name ensures that the feature's guid is unique across all ZFS
implementations. When unsupported features are encountered on a pool they will
be identified by their guids.
Refer to the documentation for the ZFS implementation that created the pool
for information about those features.
.Pp
Each supported feature also has a short name.
By convention a feature's short name is the portion of its guid which follows
the ':' (e.g.
.Sy com.example:feature_name
would have the short name
.Sy feature_name ),
however a feature's short name may differ across ZFS implementations if
following the convention would result in name conflicts.
.Ss Feature states
Features can be in one of three states:
.Bl -tag -width "XXXXXXXX"
.It Sy active
This feature's on\-disk format changes are in effect on the pool.
Support for this feature is required to import the pool in read\-write mode.
If this feature is not read-only compatible, support is also required to
import the pool in read\-only mode (see "Read\-only compatibility").
.It Sy enabled
An administrator has marked this feature as enabled on the pool, but the
feature's on\-disk format changes have not been made yet.
The pool can still be imported by software that does not support this feature,
but changes may be made to the on\-disk format at any time which will move
the feature to the
.Sy active
state.
Some features may support returning to the
.Sy enabled
state after becoming
.Sy active .
See feature\-specific documentation for details.
.It Sy disabled
This feature's on\-disk format changes have not been made and will not be made
unless an administrator moves the feature to the
.Sy enabled
state.
Features cannot be disabled once they have been enabled.
.El
.Pp
The state of supported features is exposed through pool properties of the form
.Sy feature@short_name .
.Ss Read\-only compatibility
Some features may make on\-disk format changes that do not interfere with other
software's ability to read from the pool.
These features are referred to as "read\-only compatible".
If all unsupported features on a pool are read\-only compatible, the pool can
be imported in read\-only mode by setting the
.Sy readonly
property during import (see
.Xr zpool 8
for details on importing pools).
.Ss Unsupported features
For each unsupported feature enabled on an imported pool a pool property
named
.Sy unsupported@feature_guid
will indicate why the import was allowed despite the unsupported feature.
Possible values for this property are:
.Bl -tag -width "XXXXXXXX"
.It Sy inactive
The feature is in the
.Sy enabled
state and therefore the pool's on\-disk format is still compatible with
software that does not support this feature.
.It Sy readonly
The feature is read\-only compatible and the pool has been imported in
read\-only mode.
.El
.Ss Feature dependencies
Some features depend on other features being enabled in order to function
properly.
Enabling a feature will automatically enable any features it depends on.
.Sh FEATURES
The following features are supported on this system:
.Bl -tag -width "XXXXXXXX"
.It Sy async_destroy
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:async_destroy"
.It GUID Ta com.delphix:async_destroy
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
Destroying a file system requires traversing all of its data in order to
return its used space to the pool.
Without
.Sy async_destroy
the file system is not fully removed until all space has been reclaimed.
If the destroy operation is interrupted by a reboot or power outage the next
attempt to open the pool will need to complete the destroy operation
synchronously.
.Pp
When
.Sy async_destroy
is enabled the file system's data will be reclaimed by a background process,
allowing the destroy operation to complete without traversing the entire file
system.
The background process is able to resume interrupted destroys after the pool
has been opened, eliminating the need to finish interrupted destroys as part
of the open operation.
The amount of space remaining to be reclaimed by the background process is
available through the
.Sy freeing
property.
.Pp
This feature is only
.Sy active
while
.Sy freeing
is non\-zero.
.It Sy empty_bpobj
.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:empty_bpobj"
.It GUID Ta com.delphix:empty_bpobj
.It READ\-ONLY COMPATIBLE Ta yes
.It DEPENDENCIES Ta none
.El
.Pp
This feature increases the performance of creating and using a large number
of snapshots of a single filesystem or volume, and also reduces the disk
space required.
.Pp
When there are many snapshots, each snapshot uses many Block Pointer Objects
.Pq bpobj's
to track blocks associated with that snapshot.
However, in common use cases, most of these bpobj's are empty.
This feature allows us to create each bpobj on-demand, thus eliminating the
empty bpobjs.
.Pp
This feature is
.Sy active
while there are any filesystems, volumes, or snapshots which were created
after enabling this feature.
.El
.Sh SEE ALSO
.Xr zpool 8
.Sh AUTHORS
This manual page is a
.Xr mdoc 7
reimplementation of the
.Tn illumos
manual page
.Em zpool-features(5) ,
modified and customized for
.Fx
and licensed under the Common Development and Distribution License
.Pq Tn CDDL .
.Pp
The
.Xr mdoc 7
implementation of this manual page was initially written by
.An Martin Matuska Aq mm@FreeBSD.org .
